1. Field of the Invention
The present invention relates to an apparatus for directly applying a high voltage to conductive paint so as to electrostatically spray-coat an object or workpiece with the paint.
2. Description of the Related Art
An electrostatic spray coating or painting apparatus for applying a high voltage to conductive paint so as to electrostatically spray-coat an object or workpiece such as a vehicle body to be coated, with conductive paint, has heretofore been provided with an insulation mechanism for electrically insulating a color changeover valve mechanism as a paint feed source and an intermediate reservoir from each other. As a typical example, there is known a device as disclosed in U.S. Pat. No. 4,771,729, for example.
In the prior art shown in FIG. 1, there is disposed an insulation mechanism 6 between a color changeover valve mechanism 2 and an intermediate reservoir 4. When conductive paint is applied to an object or workpiece from a spray gun 8, the insulation mechanism 6 is activated to electrically insulate the color changeover valve mechanism 2 and the intermediate reservoir 4 from each other. The insulation mechanism 6 has an electrically-insulated line 10, which has a paint inlet and a paint outlet to which two-way changeover valves 12a, 12b are respectively connected. The insulated line 10 has upper and lower end portions to which two-way changeover valves 14a, 14b for introducing cleaning fluid or liquid into the insulated line 10 and for discharging the same therefrom, respectively, and two-way changeover valves 16a, 16b for introducing air into the insulated line 10 and for discharging the same therefrom, respectively, are connected.
In the prior art referred to above, however, the insulation mechanism 6 includes six two-way changeover valves 12a, 12b, 14a, 14b, 16a and 16b, and lines connected to given positions of the insulated line 10, for disposing the two-way changeover valves 14a, 14b, 16a and 16b. Therefore, the prior art has the following problem. More specifically, it is necessary to electrically insulate the two-way changeover valve 12a from the two-way changeover valve 12b and to reliably electrically separate the two-way changeover valve 12b from the two-way changeover valves 14b, 16b. Further, the insulation mechanism 6 and the entire structure of the electrostatic spray-painting apparatus with the insulation mechanism 6 incorporated therein are both extremely large and complex in addition to an increase in the number of changeover valves (two-way changeover valves) to be used. It is also hard to clean a path between the two-way changeover valves 12a and 14a and a path between the two-way changeover valves 12b and 16b. Further, cleaning liquid tends to remain in these paths, thereby requiring much drying time and providing unstable electrical insulation.
As this type of art, there is also known a paint color changeover system disclosed in Japanese Patent Application Laid-Open No. 2-2885, for example.
According to the disclosure, water-based conductive paint is first introduced into an intermediate reservoir from a paint feed source via an electrically-insulated line (insulated portion). Thereafter, the insulated line is washed and then dried to produce an electrically-insulated state (referred to as a "voltage block"). Thus, under the condition that current is being prevented from leaking to the paint feed source, the conductive paint is supplied to a spray gun from the intermediate reservoir, and a high voltage is directly applied to the conductive paint so as to electrostatically spray-coat an object or workpiece with the so-processed conductive paint.
In the disclosure referred to above, when it is desired to clean the insulated line, deionized water is used to prevent the current from leaking. It is, however, not possible to completely prevent the current from leaking even when deionized water is used. When the deionized water remains in the insulated line in the form of a thin film, current leakage is induced. Therefore, a high voltage cannot be applied to water-based coating or paint unless the deionized water employed in a cleaning process is completely dried. Thus, the formation of the voltage block will take much time, thereby impairing the efficiency of the entire spray painting process.
In order to produce the voltage block, either water or a solution obtained by mixing water with 5%-10% hydrophilic solvents is used as a cleaning liquid. However, the insulated line is washed with the cleaning liquid composed principally of the water, thereby causing a problem in detergency.
In the above disclosure as well, the insulated line is cleaned and then dried to produce the voltage block. Therefore, resin components in the water-based conductive paint tend to remain in the inner wall of the insulated line and adhere thereto when the process for producing the voltage block is repeatedly carried out. As a consequence, an undesired flow of electricity over or through the resin components adhered to the inner wall thereof is produced, and hence a voltage blocking effect is insufficient or incapable of being completely achieved. In addition, the resin components are separated from the inner wall so as to be delivered from the spray gun. As a consequence, there is risk that the resin components may be applied to the object or workpiece, thereby causing the problem of a painting failure.